How to secure an enterprise scale ACM Private CA hierarchy for automotive and manufacturing

In this post, we show how you can use the AWS Certificate Manager Private Certificate Authority (ACM Private CA) to help follow security best practices when you build a CA hierarchy. This blog post walks through certificate authority (CA) lifecycle management topics, including an architecture overview, centralized security, separation of duties, certificate issuance auditing, and certificate sharing by means of templates. These topics provide best practices surrounding your ACM Private CA hierarchy so that you can build the right CA hierarchy for your organization.

With ACM Private CA, you can create private certificate authority hierarchies, including root and subordinate CAs, without the upfront investment and ongoing maintenance costs of operating your own private CA. You can issue certificates for authenticating internal users, computers, applications, services, servers or other devices, and code signing.

This post includes the following Amazon Web Services (AWS) services:

Solution overview

In this blog post, you’ll see an example automotive manufacturing company and their supplier companies. Each will have associated AWS accounts, which we will call Manufacturer Account(s) and Supplier Account(s), respectively.

Automotive manufacturing companies usually have modules that come from different suppliers. Modules, in the automotive context, are embedded systems that control electrical systems in the vehicle. These modules might be interconnected throughout the in-vehicle network or provide connectivity external to the vehicle, for example, for navigation or sending telemetry to off-board systems.

The architecture needs to allow the Manufacturer to retain control of their CA hierarchy, while giving their external Suppliers limited access to sign the certificates on these modules with the Manufacturer’s CA hierarchy. The architecture we provide here gives you the basic information you need to cover the following objectives:

Creation of accounts that logically separate CAs in a hierarchy
IAM role creation for specific personas to manage the CA lifecycle
Auditing the CA hierarchy by using audit reports
Cross-account sharing by using AWS RAM with certificate template scoping

Architecture overview

Figure 1 shows the solution architecture.

Figure 1: Multi-account certificate authority hierarchy using ACM Private CA

The Manufacturer has two categories of AWS accounts:

A dedicated account to hold the Manufacturer’s root CA
An account to hold their subordinate CA

Note: The diagram shows two subordinate CAs in the Manufacturer account. However, depending on your security needs, you can have a subordinate CA per account per supplier.

Additionally, each Supplier has one AWS account. These accounts will have the Manufacturer’s subordinate CA shared by using AWS RAM. The Manufacturer will have a subordinate CA for each Supplier.

Logically separate accounts

In order to minimize the scope of impact and scope users to actions within their duties, it’s critical that you logically separate AWS accounts based on workload within the CA hierarchy. The following section shows a recommendation for how to do that.

AWS account that holds the root CA

You, the Manufacturer, should place the ACM Private root CA within its own dedicated AWS account to segment and tightly control access to the root CA. This limits access at the account level and only uses the dedicated account for a single purpose: holding the root CA for your organization. This account will only have access from IAM principals that maintain the CA hierarchy through a federation service like AWS Single Sign-On (AWS SSO) or direct federation to IAM through an existing identity provider. This account also has AWS CloudTrail enabled and configured for business-specific alerting, including actions like creation, updating, or deletion of the root CA.

AWS account that holds the subordinate CAs

You, the Manufacturer, will have a dedicated account where the entire CA hierarchy below the root will be located. You should have a separate subordinate CA for each Supplier, and in some cases a separate subordinate CA for each hardware module the Supplier is building. The subordinate CAs can issue certificates for specific hardware modules within the Supplier account.

This Manufacturer account shares each subordinate CA to the respective Supplier’s AWS account by using AWS RAM. This provides joint control to the shared subordinate CA, creating isolation between individual Suppliers. AWS RAM allows Suppliers to control certificate issuance and revocation if this is allowed by the Manufacturer. Each Supplier is only shared certificate provisioning access through AWS RAM configuration, which means that you can tightly monitor and revoke access through AWS RAM. Given this sharing through AWS RAM, the Suppliers don’t have access to modify or delete the CA hierarchy itself and can only provision certificates from it.

Supplier AWS account(s)

These AWS accounts are owned by each respective Supplier. For example, you might partner with radio, navigation system, and telemetry suppliers. Each Supplier would have their own AWS account, which they control. The Supplier accepts an invitation from the manufacturer through AWS RAM, sharing the subordinate CA. The subordinate is allowed to take only certain actions, based on how the Manufacturer configured the share (more on this later in the post).

Separation of duties by means of IAM role creation

In order to follow least privilege best practices when you create a CA hierarchy with ACM Private CA, you must create IAM roles that are specific to each job function. The recommended method is to separate administrator and certificate issuer roles.

For this automotive manufacturing use case, we recommend the following roles:

Manufacturer IAM roles:
- A CA admin role with CA disable permission
- A CA admin role with CA delete permission
Supplier certificate issuer IAM role:
- Certificate issuing roles managed by attribute-based access control (ABAC) for developers

Manufacturer IAM role overview

In this flow, one IAM role is able to disable the CA, and a second principal can delete the CA. This enables two-person control for this highly privileged action—meaning that you need a two-person quorum to rotate the CA certificate.

Day-to-day CA admin policy (with CA disable)

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Action": [
                "acm-pca:ImportCertificateAuthorityCertificate",
                "acm-pca:DeletePolicy",
                "acm-pca:PutPolicy",
                "acm-pca:TagCertificateAuthority",
                "acm-pca:ListTags",
                "acm-pca:GetCertificate",
                "acm-pca:CreateCertificateAuthority",
                "acm-pca:ListCertificateAuthorities",
                "acm-pca:UntagCertificateAuthority",
                "acm-pca:GetCertificateAuthorityCertificate",
                "acm-pca:RevokeCertificate",
                "acm-pca:UpdateCertificateAuthority",
                "acm-pca:GetPolicy",
                "acm-pca:IssueCertificate",
                "acm-pca:DescribeCertificateAuthorityAuditReport",
                "acm-pca:CreateCertificateAuthorityAuditReport",
                "acm-pca:RestoreCertificateAuthority",
                "acm-pca:GetCertificateAuthorityCsr",
                "acm-pca:DeletePermission",
                "acm-pca:DescribeCertificateAuthority",
                "acm-pca:CreatePermission",
                "acm-pca:ListPermissions"
            ],
            "Resource": “*”
        },
        {
            "Effect": "Deny",
            "Action": [
                "acm-pca:DeleteCertificateAuthority"
            ],
            "Resource": <Enter Root CA ARN Here>
        }
    ]
}

Privileged CA admin policy (with CA delete)

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Action": [
                "acm-pca:ImportCertificateAuthorityCertificate",
                "acm-pca:DeletePolicy",
                "acm-pca:PutPolicy",
                "acm-pca:TagCertificateAuthority",
                "acm-pca:ListTags",
                "acm-pca:GetCertificate",
                "acm-pca:UntagCertificateAuthority",
                "acm-pca:GetCertificateAuthorityCertificate",
                "acm-pca:RevokeCertificate",
                "acm-pca:GetPolicy",
    "acm-pca:CreateCertificateAuthority",
                "acm-pca:ListCertificateAuthorities",
                "acm-pca:DescribeCertificateAuthorityAuditReport",
                "acm-pca:CreateCertificateAuthorityAuditReport",
                "acm-pca:RestoreCertificateAuthority",
                "acm-pca:GetCertificateAuthorityCsr",
                "acm-pca:DeletePermission",
    "acm-pca:IssueCertificate",
                "acm-pca:DescribeCertificateAuthority",
                "acm-pca:CreatePermission",
                "acm-pca:ListPermissions",
                "acm-pca:DeleteCertificateAuthority"
            ],
            "Resource": “*”
        },
        {
            "Effect": "Deny",
            "Action": [
                "acm-pca:UpdateCertificateAuthority"
            ],
            "Resource": <Enter Root CA ARN Here>
        }
    ]
}

We recommend that you, the Manufacturer, create a two-person process for highly privileged events like CA certificate rotation ceremonies. The preceding policies serve two purposes. First, they allow you to designate separation of management duties between day-to-day CA admin tasks and infrequent root CA rotation ceremonies. The day-to-day CA admin policy allows all ACM Private CA actions except the ability to delete the root CA. This is because the day-to-day CA admin should not be deleting the root CA. Meanwhile, the privileged CA admin policy has the ability to call DeleteCertificateAuthority. However, in order to call DeleteCertificateAuthority, you first need to have the day-to-day CA admin role disable the root CA.

This means that both roles listed here are necessary to perform a root CA deletion for a rotation or replacement ceremony. This arrangement creates a way to control the deletion of the CA resource by requiring two separate actors to disable and delete. It’s crucial that the two roles are assumed by two different people at the identity provider. Having one person assume both of these roles negates the increased security created by each role.

You might also consider enforcing tagging of CAs at the organization level so that each new CA has relevant tags. The blog post Securing resource tags used for authorization using a service control policy in AWS Organizations illustrates in detail how to secure tags using service control policies (SCPs), so that only authorized users can modify tags.

Supplier IAM role overview

Your Suppliers should also follow least privilege when creating IAM roles within their own accounts. However, as we’ll see in the Cross-account sharing by using AWS RAM section, even if the Suppliers don’t follow best practices, the Manufacturer’s ACM Private CA hierarchy is still isolated and secure.

That being said, here are common IAM roles that your Suppliers should create within their own accounts:

Developers who provision certificates for development and QA workloads
Developers who provision certificates for production

These certificate issuing roles give the Supplier the ability to issue end-entity certificates from the CA hierarchy. In this use case, the Supplier needs two different levels of permissions: non-production certificates and production certificates. To simplify the roles within IAM, the Supplier decided to use ABAC. These ABAC policies allow operations when the principal’s tag matches the resource tag. Because the Supplier has many similar policies, each with a different set of users, they use ABAC to create a single IAM policy that uses principal tags rather than creating multiple slightly different IAM policies.

Certificate issuing policy that uses ABAC

{
	"Version": "2012-10-17",
	"Statement": [
	{
		"Effect": "Allow",
		"Action": [
			"acm-pca:IssueCertificate",
			"acm-pca:ListTags",
			"acm-pca:GetCertificate",
			"acm-pca:ListCertificateAuthorities"
		],
		"Resource": "*",
		"Condition": {
			"StringEquals": {
				"aws:ResourceTag/access-project": "${aws:PrincipalTag/access-project}",
				"aws:ResourceTag/access-team": "${aws:PrincipalTag/access-team}"
			}
		}
	}
}

This single policy enables all personas to be scoped to least privilege access. If you look at the Condition portion of the IAM policy, you can see the power of ABAC. This condition verifies that the PrincipalTag matches the ResourceTag. The Supplier is federating into IAM roles through AWS SSO and tagging the Supplier’s principals within its selected identity providers.

Because you as the Manufacturer have tagged the subordinate CAs that are shared with the Supplier, the Supplier can use identity provider (IdP) attributes as tags to simplify the Supplier’s IAM strategy. In this example, the Supplier configures each relevant user in the IdP with the attribute (tag) key: access-team. This tag matches the tagging strategy used by the Manufacturer. Here’s the mapping for each persona within the use case:

Dev environment:
- access-team: DevTeam
Production environment:
- access-team: ProdTeam

You can choose to add or remove tags depending on your use case, and the preceding scenario serves as a simple example. This offloads the need to create new IAM policies as the number of subordinate CAs grow. If you decide to use ABAC, make sure that you require both principal tagging and resource tagging upon creation of each, because these tags become your authorization mechanism.

CA lifecycle: Audit report published by the Manufacturer

In terms of auditing and monitoring, we recommend that the Manufacturer have a mechanism to track how many certificates were issued for a specific Supplier or module. Within the Manufacturer accounts, you can generate audit reports through the console or CLI. This allows you, the manufacturer, to gather metrics on certificate issuance and revocation. Following is an example of a certificate issuance.

Figure 2: Audit report output for certificate issuance

For more information on generating an audit report, see Using audit reports with your private CA.

Cross-account sharing by using AWS RAM

With AWS RAM, you can share CAs with another account. We recommend that you, as a Manufacturer, use AWS RAM to share CAs with Suppliers so that they can issue certificates without administrator access to the CA. This arrangement allows you as the Manufacturer to more easily limit and revoke access if you change Suppliers. The Suppliers can create certificates through the ACM console or through the CLI, API, or AWS CloudFormation. Manufacturers are only sharing the ability to create, manage, bind, and export certificates from the CA hierarchy. The CA hierarchy itself is contained within the Manufacturers’ accounts, and not within the Suppliers’ accounts. By using AWS RAM, the Suppliers don’t have any administrator access to the CA hierarchy. From a cost perspective, you can centrally control and monitor the costs of your private CA hierarchy without having to deal with cost-sharing across Suppliers.

Refer to How to use AWS RAM to share your ACM Private CA cross-account for a full walkthrough on how to use RAM with ACM Private CA.

Certificate templates with AWS RAM managed permissions

AWS RAM has the ability to create managed permissions in order to define the actions that can be performed on shared resources. For each shareable resource type, you can use AWS RAM managed permissions to define which permissions to grant to whom for shared resource types that support additional managed permissions. This means that when you use AWS RAM to share a resource (in this case ACM Private CA), you can now specify which IAM actions can take place on that resource. AWS RAM managed permissions integrate with the following ACM Private CA certificate templates:

Permission 1: BlankEndEntityCertificate_APICSRPassthrough
Permission 2: EndEntityClientAuthCertificate
Permission 3: EndEntityServerAuthCertificate
Permission 4: subordinatesubordinateCACertificate_PathLen0
Permission 5: RevokeCertificate

These five certificate templates allow a Manufacturer to scope its Suppliers to the certificate template provisioning level. This means that you can limit which certificate templates can be issued by the Suppliers.

Let’s assume you have a Supplier that is supplying a module that has infotainment media capability, and you, the manufacturer, want the Supplier to provision the end-entity client certificate but you don’t want them to be able to revoke that certificate. You can use AWS RAM managed permissions to scope that Supplier’s shared private CA to allow the EndEntityClientAuthCertificate issuance template, which implicitly denies RevokeCertificate template actions. This further scopes down what the Supplier is authorized to issue on the shared CA, gives the responsibility for revoking infotainment device certificates to the Manufacturer, but still allows the Supplier to load devices with a certificate upon creation.

Example of creating a resource share in AWS RAM by using the AWS CLI

This walkthrough shows you the general process of sharing a private CA by using AWS RAM and then accepting that shared resource in the partner account.

Create your shared resource in AWS RAM from the Manufacturer subordinate CA account. Notice that in the example that follows, we selected one of the certificate templates within the managed permissions option. This limits the shared CA so that it can only issue certain types of certificate templates.

Note: Replace the <variable> placeholders with your own values.
```
aws ram create-resource-share
		--name Shared_Private_CA
		--resource-arns arn:aws:acm-pca:<region:111122223333>:certificate-authority/<xxxx-xxxx-xxxx-xxxx-example>
		--permission-arns "arn:aws:ram::aws:permission/<AWSRAMBlankEndEntityCertificateAPICSRPassthroughIssuanceCertificateAuthority>"
		--principals <444455556666>
```
From the Supplier account, the Supplier administrator will accept the resource. Follow How to use AWS RAM to share your ACM Private CA cross-account to complete the shared resource acceptance and issue an end entity certificate.

Conclusion

In this blog post, you learned about the various considerations for building a secure public key infrastructure (PKI) hierarchy by using ACM Private CA through an example customer’s prescriptive setup. You learned how you can use AWS RAM to share CAs across accounts easily and securely. You also learned about sharing specific CAs through the ability to define permissions to specific principals across accounts, allowing for granular control of permissions on principals that might act on those resources.

The main takeaways of this post are how to create least privileged roles within IAM in order to scope down the activities of each persona and limit the potential scope of impact for your organization’s private CA hierarchy. Although these best practices are specific to manufacturer business requirements, you can alter them based on your business needs. With the managed permissions in AWS RAM, you can further scope down the actions that principals can perform with your CA by limiting the certificate templates allowed on that CA when you share it. Using all of these tools, you can help your PKI hierarchy to have a high level of security. To learn more, see the other ACM Private CA posts on the AWS Security Blog.

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